Individual "configure mega" pages behave strangely

There are two significant UX problems with the settings pages for each menus' Megamenu configuration:

1. The breadcrumbs trail is missing.
2. Clicking "Save Configuration" should redirect the user to the main Megamenu config page, rather than leaving the user on the same config page. There's no "your settings were saved" message, either.

I've included a patch that remedies these problems. I also fixed a logic error in the code that checks for custom skin names not being plain text. It'll now actually fire the appropriate form_set_error() call if the user inputs something that isn't plain text.